The present invention concerns a method for starting an internal combustion engine, as well as a starter system for an internal combustion engine.
It is known from practice that internal combustion engines (for example, in vehicles), cannot be started from their own power. They must initially be cranked by an external power source (the so-called starter), and accelerated to the engine speed required for starting of the internal combustion engine. Only then can they continue to run under their own power.
A battery-fed DC motor is often used as the electric starter in vehicles. This motor transfers the necessary starting torque to the crankshaft of the internal combustion engine via a drive pinion that engages in a toothed ring on the disk flywheel. A starter system with an electric starter motor, whose rotor sits directly on the crankshaft of the internal combustion engine and is connected to rotate in unison with it, is also known from DE 44 06 481 A1. With this type of arrangement, the weight of the rotor of the electric machine is simultaneously used as flywheel mass.
An improved starter system of this type is also known from EP 0 569 347 A2 and WO 91/16538.
The starting torque of an internal combustion engine and the minimum starting speed depend, among other things, on engine type, working volume, number of cylinders, bearing friction, compression and mixture preparation and, above all, on temperature. The section of the operating process, in which the cylinder or cylinders of the engine are situated during starting, is also significant for compression of an internal combustion engine and therefore for its readiness to start. Thus, for example, the compression of a cylinder situated in the compression stroke has an unfavorable effect on starting behavior, because it opposes the starter with increased torque right at the beginning of starting. Thus far in the prior art this variable has not been adequately considered. Known starters in each case had to be designed according to power, so that the internal combustion engine can be started under all conditions.
In the disclosed method for starting an internal combustion engine, the crankshaft of the internal combustion engine is accelerated at least to a speed (so-called starting speed) necessary for starting the internal combustion engine. An electric machine is used for this acceleration, whose rotor acts directly on the crankshaft or via a transmission connected in between. The crankshaft is also brought to a stipulated crank angle position or stipulated crank angle (hereafter xe2x80x9cstarting anglexe2x80x9d) by means of the electric machine for the starting process and the internal combustion engine is started from this starting angle. The power required for starting is taken at least partially from a capacitor accumulator. The actual starting process of the internal combustion engine can then begin from a favorable starting angle and is additionally fed, at least partially, from the capacitor accumulator (not fully from a starter battery, as usual), which can deliver the necessary electrical starting power much more quickly than an ordinary battery. Moreover, a capacitor accumulator is much less temperature-sensitive than an electrochemical battery, so that, even at very low temperatures, problem-free starting is possible.
Charging of the capacitor accumulator can occur in different ways. One possibility comprises charging the capacitor accumulator only before starting from a starter battery. The command that triggers the adjustment process of the crankshaft starting angle is preferably simultaneously used as signal to charge the capacitor accumulator from the starter battery. Starting of the internal combustion engine can then occur without any waiting time.
A disclosed starter system for an internal combustion engine includes: an electric machine, whose rotor is connected directly to rotate in unison with the crankshaft of the internal combustion engine or via a transmission connected in between, in order to accelerate the internal combustion engine at least to a speed (starting speed) necessary for starting; means to record and/or derive the crank angle of the internal combustion engine; a control device that controls the electric machine, so that the crank angle of the internal combustion engine is brought to a stipulated starting angle for the starting process; and a capacitor accumulator (for example, a so-called intermediate circuit accumulator), which at least partially supplies the power required for starting. The capacitor accumulator can preferably also be a combination of electrical capacitor elements and electrochemical battery elements.
The inventors recognized that the position of the crankshaft at the beginning of starting is of considerable importance for the starting behavior of an internal combustion engine. Based on this recognition, the inventors further recognized that, by influencing the crank angle before the actual starting process, as well as the type of starting power supply, a significant improvement in starting behavior of an internal combustion engine can be achieved. By means of the electric machine (for example, a so-called crankshaft starter with a rotor connected to rotate in unison directly with the crankshaft), it is possible to apply the torques necessary for adjustment of a desired starting angle in both directions of rotation of the crankshaft and with high accuracy. In this manner, an unfavorable crankshaft position at the beginning of starting is avoided, for example, when one or more cylinders of an internal combustion engine compress right at the beginning, and starting can thus be achieved with reduced starting power. In terms of the device, the starter system has a control device for this purpose, which, knowing the instantaneous crank angle, controls the rotor of the electric machine (optionally with consideration of the transmission ratio between the rotor and crankshaft), so that the crankshaft is brought to the desired starting angle.
Use of the disclosed starter system is advantageous both in spark ignition engines and diesel engines (for example, four-stroke engines with manifold injection or with direct injection), which are designed for use in passenger cars.
In a preferred variant, the crank angle at which the starting torque to be applied by the electric machine is lower at the beginning of the starting process than in the known starter systems is chosen as the starting angle. In an internal combustion engine operating, for example, in the four-stroke method, the cylinder pressure, and therefore the compression to be overcome by a starter, increases during a compression stroke and reaches its maximum roughly in the region of top dead center. If, in a preferred variant for a four-stroke internal combustion engine, the crank angle for the next start is set at the end of the compression stroke, preferably in a region right after top dead center, at the beginning of starting, the starter need only overcome the relatively low-compression suction stroke of the internal combustion engine. Moreover, almost two full revolutions remain for the starter at the beginning of starting, in order to build up sufficient starting power to overcome the next compression stroke. This is particularly favorable in a cold start.
In another variant, the crank angle at which the starting time (i.e., the time from the beginning of starting to starting of the internal combustion engine), is reduced to a minimum is chosen as starting angle. In a four-stroke internal combustion engine with manifold injection, this is preferably the crank angle position at the beginning of the suction stroke, with particular preference in the intersection region between the exhaust and suction stroke. On the other hand, in a four-stroke internal combustion engine with direct injection, the crank angle position is preferably at the end of the suction stroke. If the internal combustion engine is also equipped with an ordinary sensor system comprising an inductive sensor and gear with reference marks to record the crank angle, the starting angle adjustment process, and thus the starting time, can also be shortened, so that the starting angle is chosen in the region right before the reference mark of the rotation angle sensor. Rotation angle recording can then be carried out without a delay right at the beginning of the starting process.
If starting can occur without any delay, this also serves for traffic safety and increases operating comfort of vehicles. Moreover, the amount of power required overall to start an internal combustion engine is then lower, which advantageously permits smaller dimensioning of the starter power accumulator.
The discussion thus far has applied equally to one-cylinder and multicylinder engines, if selection of the crankshaft starting angle is adjusted to that cylinder of a multicylinder engine that is ignited first. Generally, the sequence in which the cylinders are ignited in succession is stipulated. However, in another variant, at least during selection of the cylinder ignited first, a deviation is made from the stipulated ignition sequence and a specific cylinder is selected for the first ignition as a function of the starting angle of the crankshaft being adjusted.
The internal combustion engine is preferably brought automatically to a starting angle favorable for the next start already during disengagement or right after disengagement of ignition of the internal combustion engine by means of the electric machine arranged in the drive train, for example, in which the electric machine has a braking or accelerating effect on the crankshaft of the running out internal combustion engine. As an alternative to this approach, the desired starting angle is set only right before the beginning of the starting process automatically, for example, in which the electric machine rotates the crankshaft of the stopped internal combustion engine forward or backward into the desired starting angle. An undesired xe2x80x9cadjustmentxe2x80x9d of the starting angle, once set, in the time between the adjustment process and the starting process is therefore ruled out. It is particularly favorable if the power required for starter operation, in conjunction with the last-named variant, is at least partially taken from a capacitor accumulator.
For adjustment of the starting angle of the crankshaft, the instantaneous crank angle is determined, compared with the value of the stipulated crankshaft starting angle in the control device and any change in crank angle also monitored. For this purpose, an angle recording integrated in the electric machine is preferably used. With particular preference, an appropriate rotation angle sensor is coordinated with the rotor of the electric machine (for example, an inductive or optical rotation angle sensor). The rotation angle of the electric machine, however, can also be determined from the magnetic reflux of the rotor in the stator of the electric machine. Since the rotor of the electric machine is connected either directly to the crankshaft of the internal combustion engine or via a transmission, the crank angle is obtained directly or by simple conversion, with consideration of the transmission ratio.
In principle, any type of electric machine which is capable of applying the necessary torques and precisely carrying out the desired crank angle adjustment is suitable for use with the disclosed starter system, be it a DC, AC, three-phase asynchronous or three-phase synchronous machine. The electric machine/motor of the starter system is preferably an electric machine functioning as a starter/generator, which preferably permanently runs with the internal combustion engine. With particular preference, the electric machine of the starter system is an inverter-controlled three-phase machine. Three-phase machine is understood to mean a machine, in which a magnetic rotating field occurs that rotates by 360xc2x0 and, in so doing, drives the rotor. The inverter receives signals from the control device and produces alternating currents with freely adjustable frequency, amplitude and phase. This type of arrangement is excellently suited for applying high torques in both directions of rotation of the crankshaft.
Embodiments and features that were outlined above, or will be outlined subsequently in conjunction with the process, naturally also apply as disclosed in conjunction with the corresponding starting system (and vice versa).
Other features and advantages are inherent in the disclosed apparatus or will become apparent to those skilled in the art from the following detailed description and its accompanying drawings.